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Another effect of high serotonin levels is demonstrated by crustacean Gammarus lacustris , which display an abnormal response to disturbances in the water by swimming up the surface and clinging to solid objects when infected with the acanthocephalan Polymorphus paradoxus (Helluy 1990). When uninfected amphipods were injected with serotonin, they displayed the same behavior. The increased serotonin levels might have influenced chemical pathways related to the precopulatory clinging of the male amphipod (Helluy 1990).
Rabies is a disease caused by the lyssa virus that affects many vertebrates, such as dogs, bats, monkeys, foxes, and humans (Easmon 2005). It is found on all continents and between 40,000 and 70,000 people die of the infection every year.
As with acanthocephalans, the rabies virus requires transmission from one host to the next for survival. One of the mechanisms of transmission is through saliva, with the current host biting the next one to transfer the virus. After the initial penetration into the new host’s body, the virus bypasses the immune system and travels through the nervous system into the brain, skin, mucous membranes, and salivary glands. The virus causes inflammation in the brain and interferes with many of its functions. The new host becomes aggressive and prone to biting, which helps further transmission of the virus.
Rabies infection proceeds through several stages. The time that it takes for the virus to reach the brain constitutes an incubation period, which can last months. Following incubation is a preliminary stage during which the individual develops various gastrointestinal and respiratory problems, such as loss of appetite, fever, nausea, and vomiting. The individual also experiences copious salivation and tear production. Then, during the neurological stage, the infected host begins to suffer from paralysis, throat spasms, fear of water, anxiety, hyperactivity, confusion, uncontrolled movement, and delirium. After the appearance of symptoms, the mortality rate is nearly one hundred percent. However, a vaccine has been developed that treats the infection if administered before or soon after the transmission (Easmon 2005).
While the acanthocephalans work hard to improve the chances of their intermediate host being eaten, the last thing that they want is for someone besides the definitive host taking advantage of the situation and getting an easy meal. Thus, the parasites also modify the behavior of their hosts in various ways to prevent consumption by non-host predators. For example, uninfected crustacean Gammarus roeseli usually resides in the benthic (i.e. bottom) areas of the lake. However, when infected by Polymorphus minutus , they appear in higher concentrations near the water surface, often clinging to floating material, in the presence of non-host predators, such as another aggressive crustacean near the bottom of the lake (Medoc et al. 2006). The parasites also improve the host’s performance when it comes to escaping non-host predators, so that infected G. roeseli have higher average and maximum swimming speeds, greater speeds during the moment of escape, and cover greater distance while fleeing. This counterintuitive effect on the host’s fitness may increase the chances of the intermediate host being eaten by an appropriate definitive host (Medoc 2007).
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